This white paper describes the
steps involved in assessing the staffing and telecom trunk requirements of a call
centre. Online calculators
are hosted at this Web site and can be used free of charge now to assist with these
calculations.

Contents

Purpose

This white paper describes the steps involved in assessing the staffing
requirements of a call centre and the estimating the number of trunks (central office
lines) required to serve a call centre for incoming calls.

Software
tools required

In order to help you with your calculations, we have provided free online traffic calculators at this Web
site which you can use now. A Windows version
is available for immediate download at 99 US Dollars and
offers increased speed, capacity and convenience.

Ansapoint is a call centre analyser which automates the design
process discussed in this paper. It is available for immediate download for just 185 US Dollars.

Description
of the design process

There are two distinct areas of design required in such an application.
The questions which must be answered are:

How many call centre agents do I need?

How many trunks do I need?

As call holding times depend upon average queuing times (which depend upon the number
of agents deployed), the two questions must be addressed in the order shown.

How many
agents to I need?

Calculating the number of agents required is a continuous process which
will require regular reassessment as the circumstances of a call center change.
Assessments may be made for each working hour of a day, and should take such factors as
marketing campaigns and daily call peaks into account.

We suggest performing a
calculation for each working hour. In order to estimate the number of agents required in a
particular hour, the following information relating to that hour is required as a minimum:

Number of calls received

Average duration of these calls

Average delay that you accept that incoming callers may experience.

Items 1 and 2 describe the incoming traffic levels and must be established from call
statistics or from estimates based on your understand of your business. Item 3 is your
performance criterion. Another performance criterion which can be used defines call
handling in terms of the percentage of calls answered within a target queuing time (e.g.
85% of calls answered within 20 seconds of ringing). This can be more meaningful and is
supported by our Windows 95 / NT product, Westbay Traffic Calculators, but is not yet
supported by our online calculators.

Wrap up time (or wrap time) is the time an agent remains unavailable to answer a call
after a call has been completed. It is usually the time taken to carry out administrative
tasks relating to a call such as entering an order on a terminal. For the purposes of
Erlang C, wrap up time should be included in average call duration.

Having established these three minimum parameters for an hour, an estimate of the
number of agents required can be made using the Erlang C Traffic Model. You can use our online calculator to work through this
example now.

Pressing the Calc button reveals that 21 agents will be required during the hour
in question. Performing this calculation using our Windows
product, Westbay Traffic Calculators is similar, but please refer to the user guide or
help system for detailed instructions.

How many
trunks do I need

Whereas the number of agents required can (and should) be dynamic, changing from hour
to hour, the number of lines required to connect a call center with a central office
exchange is fixed (at least in traditional circuit switched technology) and must cater for
the maximum anticipated traffic levels which will be encountered. Engineering the number
of lines required is known as dimensioning a trunk group.

The Erlang B traffic
model can be used to estimate the number of lines required. This traffic model requires
the following inputs:

Busy Hour Traffic

Blocking

Busy Hour Traffic
This figure represents the quantity of traffic expressed in a unit called Erlangs. For the
purposes of these calculations, 1 Erlang can be considered equivalent to 1 hour of calls.

The busiest hour must always be used for busy hour traffic calculations. But, wrap up
time is not included. In working out the number of lines required, the busy hour traffic
must be based on the duration of the calls and the queuing times as these account for
trunk occupancy; wrap up time does not occupy a trunk.

The resulting figure shows the total trunk occupancy in hours, including the average
delay period during which calls are being queued in an ACD and occupying trunks.

So, the busy hour traffic figure would be:

BHT = [ 160 + 25 ] * 350 / 3600
BHT = 17.986 Erlangs

It is important to note that the busy hour traffic figure should represent the busiest
traffic load a call centre will ever be offered. The trunk group being designed must be
large enough to cater not just for today's peak, but for every peak. Therefore, extreme
caution should be exercised when calculating BHT.

BlockingThe blocking figure describes the calls which cannot be completed because
insufficient lines have been provided. A figure of 0.01 means that 1% of calls would be
blocked; this is a normal figure to use in traffic engineering. For some applications,
0.03 (3%) blocking is used.

Having established these two parameters, an estimate of the number of lines required
can be made using the Erlang B Traffic Model. You can use our online calculator to work through this example now.

BHT = 17.986 Erlangs

Blocking = 0.01

Pressing the Calc button reveals that 28 lines will be required during the hour
in question. Performing this calculation using our Windows
95, Westbay Traffic Calculators is similar, but please refer to the user guide or help
system for detailed instructions.

Reasons
for caution

The Erlang B and C traffic models make certain assumptions about the
nature of the call arrivals. Amongst them is the assumption that call arrivals are random
(Poisson arrivals). Although this is quite reasonable in most applications, it can cause
inaccurate results when there is a sudden peak of calls.

This type of peak can be
produced by a radio or television advertisement being shown (which can often be the reason
for a call centre's existence in the first place!) Where drastic call peaks are expected,
over-engineering of trunks and call center agents should always be carried out - always be
on the safe side!

The Erlang C traffic model does not take abandoned calls into account, but if your call
center is engineered correctly, this should not be a factor. It may cause a problem when
attempting to use Erlang C to analyse an existing call centre with poor performance.

This document
should not be viewed as a consultative document. It is the readers' responsibility to
ensure that the most appropriate telecommunications strategy is applied to his or her
business. No liability is accepted by the authors for omission or error.